Because the glass components PbO and As.sub.2 O.sub.3 have entered public debate as environmental pollutants, there is a demand by the manufacturers of optical equipment for 10 PbO-free and preferably also As.sub.2 O.sub.3 -free glasses having the optical properties substantially the same as glasses having these oxides.
Simple replacement of lead oxide by one or more constituents generally does not succeed in reproducing the desired optical and glass-engineering properties affected by PbO. Instead, new developments or extensive changes in the glass composition are necessary.
The patent literature reveals some specifications which have already described lead-free glasses having optical values from the stated range and having similar compositions. However, the glasses have a wide variety of disadvantages.
Both the glasses of the unexamined Japanese publication JP 62-100 449 A, having refractive indexes n.sub.d of from 1.64 to 1.88, and those of JP 60-22 133 8 A, having refractive indexes n.sub.d of from 1.62 to 1.85, contain Li.sub.2 O as a required constituent, contents of up to 15 and 12% by weight respectively being possible. Li.sub.2 O-containing glasses of this type are very susceptible to crystallization. The same also applies to the glasses of JP 63-170 247 A, which contain at least 5 mol-% of Li.sub.2 O.
The glasses of the first-mentioned specification also contain from 2 to 20% by weight of Sb.sub.2 O.sub.3. Use of this component in such amounts makes increased safety precautions necessary during production.
DE-A 35 34 575 Al relates to glasses having refractive indexes n.sub.d.ltoreq.1.59 for eyeglass lenses having color contrast amplification. The glasses, which vary greatly in their composition, contain various coloring components in a base glass containing up to 33% by weight of Nd.sub.2 O.sub.3.
The glasses of JP 53-90 312 A have relatively low refractive indexes of n.sub.d.ltoreq.1.67. They contain at least 3% by weight of the glass former Al.sub.2 O.sub.3 in order to counter the tendency toward crystallization caused by the relatively high content of alkaline earth metal oxides, but this has a disadvantageous effect on the melting properties and the "length" of the glass. The latter also applies to the Al.sub.2 O.sub.3 - and F-containing glasses of JP 1-133956 A having low refractive indexes. The glasses of JP 62-126 33 A, which likewise have low refractive indexes, contain expensive Cs.sub.2 O as a required component and have high proportions of SiO.sub.2 of from 30 to 75 mol-%.
German Patent Specification DE 32 01 344 C2 describes high-refractive index glasses having a refractive index n.sub.d.ltoreq.1.79 in which BaO is only an optional component and which contain high proportions of at least 13% by weight of the expensive component La2O.sub.3, which promotes crystallization at these rather high contents.
JP 52-25812 A and JP 58-46459 B2 describe flint glasses from the SiO.sub.2 --Nb.sub.2 O.sub.5 --TiO.sub.2 --R.sub.2 O(--RO) system, where some of the Nb.sub.2 O.sub.5 can be replaced by Ta.sub.2 O.sub.5 and some of the SiO.sub.2 can be replaced by B.sub.2 O.sub.3. The B.sub.2 O.sub.3 content should not exceed 10% by weight and 5% by weight respectively (based on all the glass) owing to crystallization phenomena.